This application is based on patent application No. 11-164773 filed in Japan, the content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an image display device for use in front of the eyes of an observer, and specifically relates to the image display device that spatially combines a virtual image of an image displayed by a display device with an external object such as a natural background or the like of a front field of view via a hologram combiner for presentation to the observer.
2. Description of the Related Art
Image display devices used in front of the eyes of an observer are either head-mounted or handheld devices, and have been developed and commercialized as products ranging from an aircraft instrument to display flight information such as altitude and airspeed, to a personal theater devices to display movies, video games, and artificial realities. There is current research on using such devices as computer displays.
Such image display devices may be of a see-through type which allows the outside world to be seen, or a closed type which does not allow the outside world to be seen. A closed type device is desirable when displaying an artificial reality, and a see-through type device is generally desirable when used as a portable device. See-through image display devices are provided with a beam combiner for combing the virtual image light with external light in addition to an image display and a viewing optical system for providing a virtual image at a location readily viewable by a observer of an image displayed by the image display.
A half-mirror, polarizing beam splitter, hologram and the like can be used as a beam combiner. Since a reflective type hologram has a high degree of wavelength selectability, it can be used to diffract only light of specific wavelength, and image light of the specific wavelength can be overlaid on external light excluding this wavelength, such that there is very little loss of external light and viewing is excellent.
Image display devices of the see-through type using a reflective-type hologram of the aforesaid characteristics as a combiner are disclosed in Japanese Laid-Open Patent Nos. 5-346508 and 10-319343.
In an image display device using a reflective-type hologram combiner, since the image light participates from the viewing side relative to the hologram combiner, the image light entering the hologram combiner and the viewing light comprising external light and the reflected light of the image light exiting the hologram combiner must be separated.
For this reason, in the image display devices of the aforesaid disclosures, there is a large amount of decentering of the hologram combiner to make possible the separation of the image light and the viewing light. However, decentering distortion is generated due to the large amount of decentering of the hologram combiner, such that an excellent virtual image cannot be provided, and wide field angle projection light cannot be provided. Furthermore, compactness of the overall device is lost because the optical axis of the optical system from the image display to the hologram combiner is a straight line.
An object of the present invention is to provide an improved see-through type image display device.
Another object of the present invention is to provide a see-through image display device capable of providing excellent wide angle images in a compact structure suited for usage modes.
These objects are attained by a image display device having the following construction: an image display device for displaying a combined image by combining an virtual image with an external object, has a reflective type hologram combiner having an optical power for forming a surface optically equivalent to the external object at a position different from the external object; an image display for displaying an image to be provided as the virtual image at the position of the surface formed by the optical power of the combiner, and a selection surface for selectively reflecting and transmitting an image light from the image display and an external light from the external object, wherein the image light is reflected by the selection surface, reflected by the hologram combiner, transmitted through the selection surface, and thereafter directed to the pupil of an observer, while an external light is transmitted through the hologram combiner, transmitted through the selection surface, and thereafter directed to the pupil of the observer.
This image display device provides both bright displayed image and external object because it has a high degree of wavelength selectability and presents to the observer an overlay of the display image and external object via a reflective type hologram combiner which has minimal effect on the transmitted light. Furthermore, the hologram combiner has optical power such that the display image is viewed at the same position as the external object. That is, an observer views the display image as a virtual image focused at infinity. In this way, the hologram combiner has an eyepiece function to enlarge the display image and direct the image to the pupil of an observer. Since provision of a separate lens and the like having an eyepiece function is unnecessary, the overall device is simplified and compact.
The selection surface is provided to bend the optical path of the image light. A bright image is provided since, in the selection surface, the image light is completely reflected and directed to the hologram, and the viewing light comprising and overlay of the displayed light and external light is completely transmitted and directed to the pupil of an observer. Since the selection surface selects luminous flux via its entrance angle, the hologram combiner is greatly decentered, and optical path separation is unnecessary. Furthermore, an excellent quality image is provided at wide field angles.
In this image display device, the number of times the image light can be reflected by the selection surface before the image light entered the hologram combiner is not limited to a single reflection, and may be reflected a plurality of times. The overall device may be rendered more compact when constructed to utilize a plurality of such reflections.